This invention relates to a basic multi-technology application of cascaded optical beam-formers or beam spoilers and, more particularly, to the use of such beam-formers in fiber-optic (FO) attenuators and switch modules using a micro-electromechanical systems MEMS-EO beam-former approach to three dimensional (3-D) beam control. In one embodiment, the invention uses the physical cascading of two 3-D beam-formers, namely, an electronically controlled optical MEMS two axis micro-mirror with optional z-axis translational control coupled with an electronically controlled EO liquid crystal device 3-D beam-former to form a robust single beam attenuation and routing module. These dual 3-D beam-former based high speed, robust, fault-tolerant FO structures can be used for routing and attenuating multiple light signals in optical networks such as wavelength division multiplexed (WDM) optical communications, distributed sensor networks, and photonic signal processing systems and can also be deployed in free-space optical applications such as laser communications, metrology instrumentation, and optical read/write data systems.
The programmable 3-D optical beam-former scanner module is a basic building block for many optical applications as it can be used to accomplish routing in fiber communications networks, photonic signal processing, distributed optical sensing, and optical controls. This 3-D scanner module can also be used to form variable optical attenuators used in building blocks for several key optical systems such as attenuators required as equalizers in wavelength division multiplexed (WDM) fiber-optic (FO) communication systems using non-uniform gain optical amplifiers. Other important applications include polarization dependent loss compensation in fiber optic networks, optical component testing, wavelength tunable receivers, and optical detector protection.
The desired features for such a 3-D beam-forming module include wide angle scans with fine angular controls, focus/defocus capability, polarization independence, low optical loss (e.g., less than 1 dB), low inter-beam crosstalk ( less than xe2x88x9230 dB), multiple simultaneous beams generation, robustness to catastrophic failure, low electrical power consumption, and simple to align low cost designs for large scale commercial production and deployment. Depending on the application, 3-D beam-former module switching speeds can range from nanoseconds to several milliseconds.
For centuries, an excellent choice for light scan control has been the use of mirrors. Mirrors provide high reflectivity over broad bandwidths, as desired in WDM systems. Today, an excellent method for making actively controlled mirrors is via micro-electromechanical system (MEMS) technology that promises to offer low cost compact optical modules via the use of low cost batch fabrication techniques similar to semiconductor electronic chip production methods. Optical MEMS technology using micro-mirrors has been previously proposed to realize fiber optic beam control modules to form attenuators and switches. In these cases, a micro-mirror acts to deflect or obstruct a single light beam in one or two dimensions, thus routing or attenuating it to a given fiber-optic channel. Both analog and digital states of the micro-mirror have been used for routing and attenuation. In analog mirror control, a micro-mirror sweeps through a continuous range of angles or translational positions. In digital micro-mirror operation, the micro-mirror has two distinct states such as +10 and xe2x88x9210 degree tilt states. Examples of such applications of the micro-mirror are described in L. Y. Lin, E. L. Goldstein, and R. W. Tkach, xe2x80x9cFree-space micromachined optical switches with submillisecond switching time for large-scale optical crossconnects,xe2x80x9d IEEE Photonics Technology Letters, Vol. 10, No. 4, pp. 525-527, April 1998; J. E. Ford and J. A. Walker, xe2x80x9cDynamic spectral power equalization using micro-opto-mechanics,xe2x80x9d IEEE Photonics Technology Letters, Vol. 10, No. 10, pp. 1440-1442, October, 1998; J. E. Ford, J. A. Walker, V. Aksyuk, and D. J. Bishop, xe2x80x9cWavelength selectable add/drop with tilting micro-mirrors,xe2x80x9d IEEE LEOS Annual Mtg., IEEE, NJ., postdeadline paperPD2.3, November, 1997, N. A. Riza and S. Sumriddetchkajom, xe2x80x9cVersatile multi-wavelength fiber-optic switch and attenuator structures using mirror manipulations,xe2x80x9d Optics Communications, Vol. 169, pp. 233-244, Oct. 1, 1999, P. Colboume et. al., xe2x80x9cVariable optical attenuator,xe2x80x9d U.S. Pat. No. 5,915,063, Jun. 22, 1999, F. H. Levinson, xe2x80x9cOptical coupling device utilizing a mirror and cantilevered arm,xe2x80x9d U.S. Pat. No. 4,626,066, Dec. 2, 1986, T. G. McDonald, xe2x80x9cUsing an asymmetric element to create a 1xc3x97N optical switch,xe2x80x9d U.S. Pat. No. 5,774,604, Jun. 30, 1998, and H. Laor, J. D"" Entremont, E. Fontenot, M. Hudson, A. Richards, and D. Krozier, xe2x80x9cPerformance of a 576xc3x97576 Optical Crossconnect,xe2x80x9d pp. 276-281, National Fiber Optic Engineers Conf. Proceedings, Sept. 26-30, 1999.
All previous micro-mirror MEMS-based optical control structures have not exploited the 3-D beam control aspect to form the FO switches and attenuators. This is because a typical mirror provides 1-D and 2-D scans, without any focus/defocus controls. It is well known that freespace propagation of beams leads to beam spreading, eventually causing loss between the fiber input-output ports. Furthermore, when input to output path distances get large (e.g.,  greater than 50 cm), slight vibrations or mechanical misalignments due to component finite tolerances or environmental conditions can cause partial loss of signal or even catastrophic failure. In particular, for the large (Nxc3x97N where N greater than 100) port count switch matrices, the input to output freespace distance are forced to be large (e.g.,  greater than 50 cm), causing a high chance for switch failure. Today, there is no mechanism for providing tolerance to the mentioned failures in an optical switch matrix as the mirrors can intrinsically provide only 2-D tilt controls.
Another problem with the previously proposed analog drive MEMS-based modules is that they require the micro-mirror to deliver both coarse beam angular deflections and fine high resolution beam alignment, leading to requiring precise analog voltage control, adding to the cost of the component. Furthermore, it is well known that mechanically actuated optical mirrors perform well across a range of large angular motions (e.g., xc2x145 degrees), but suffer greatly for fine tweaking (e.g.,  less than xc2x10.5 degrees) due to inertia limits . In addition, the mechanics and electronics required for fine mirror control become large, power consuming, heavy, and expensive. On the other hand, electro-optic (EO) materials such as liquid crystals (LC""s) can be used to form high resolution low power 3-D optical beam-formers using milliwatt level electrical power with small, lightweight and low cost designs. Such a 3-D EO beam-former was described in N. A. Riza and Shifu Yuan, xe2x80x9cDemonstration of a liquid crystal adaptive alignment tweaker for high speed infrared band fiber-fed free-space systems,xe2x80x9d Optical Engineering, Vol. 37, No. 6, June, 1998. Also in G. D. Love, xe2x80x9cLiquid Crystal Phase Modulator for unpolarized light,xe2x80x9d Applied Optics, pp. 2222-2223, Vol. 32, No. 13, May 1, 1993 and N. A. Riza and Shifu Yuan, xe2x80x9cRobust Packaging of Photonic RF Modules using Ultra-Thin Adaptive Optical Interconnect Devices,xe2x80x9d SPIE Conf. on Optical Technology for Microwave Applications VIII, Vol. 3160, pp. 170-177, San Diego, August 1997, the NLC device is described in a reflective arrangement with a fixed mirror. Furthermore, G. D. Love proposes a setup for unpolarized light for astronomical image sharpening where typically the optical receiving apertures are very large (several meters diameter) telescopes implying that the adaptive optics is also large with very high (e.g., a million pixel) space bandwidth product processing.
The focus of this application is optical fiber-based polarized light control for small aperture (a millimeter or so diameter) 3-D beam-forming and spoiling to accomplish robust FO switching and attenuation. The optical beam control module invention in this patent application solves the coarse-fine control dilemma for FO attenuators and switches via the deployment of a dual 3-D beam-former cascade using both EO and MEMS technologies. The proposed module provides alignment tolerance, simplicity of control, and reduction of component failure probability, among other features required for a successful deployable module. The proposed invention is based on the 3-D beam spoiler concept as suggested by N. A. Riza in N. A. Riza and N. Madamopoulos, xe2x80x9cSynchronous amplitude and time control for an optimum dynamic range variable photonic delay line,xe2x80x9d Applied Optics, Vol. 38, No. 11, pp. 2309-2318, Apr. 10, 1999. The proposed 3-D beam control module has both transmissive and reflective designs. In particular, the reflective design exploits the beam-forming capabilities of a mirror beam-former that is used to form the reflective architecture leading to a powerful dual cascade 3-D beam-former module.
In this invention, I use a module design employing a cascade of two 3-D optical beam-formers or spoilers. In the preferred embodiment, the EO 3-D beam-former is merged with a mirror such as a MEMS-based two axis mirror to form the desired 3-D optical beam-forming control module. This module becomes the required optical module for solving the problems associated with previous micro-mirror or liquid crystal (LC) based beam-formers. The invention uses this module to form failure and alignment tolerant switches and attenuators with built-in robustness that exploits the best features of both the optical MEMS and EO LC technologies, while suppressing or eliminating the weakness of each of the technologies. The optical MEMS-EO merger realizes a powerful 3-D optical beam control module.